1. Technical Field
The present invention relates to a gallium nitride (GaN) single crystal substrate and a method of processing the surface of the same.
2. Discussion of Related Art
Generally, gallium nitride (GaN) has appropriate properties as optical devices and high-temperature and high-power devices because of a large energy band gap, a high binding force between atoms, and a high thermal conductivity. Accordingly, GaN-based semiconductor chemical compounds are variously used as materials for fabricating optical devices. Specifically, blue and green light emitting devices fabricated by using GaN are applied to a wide range of fields, such as multimedia, traffic lights, interior lights, high-density light sources, high-resolution power systems, information and communication devices, and the like.
A gallium nitride (GaN) substrate is fabricated by growing GaN on a sapphire (Al2O3) substrate which is heterogeneous. However, since gallium nitride and sapphire are different from each other in the lattice constant and the coefficient of thermal expansion, when a gallium nitride layer is formed on the sapphire substrate, a high-density crystal defect may occur. Accordingly, when the gallium nitride layer is separated from the sapphire substrate, the gallium nitride layer may become bent or cracked.
FIG. 1 is a sectional view of a base substrate 10 and a gallium nitride layer 12 grown on the base substrate 10, and FIG. 2 is a sectional view of the gallium nitride layer 12 separated from the base substrate.
Referring to FIGS. 1 and 2, the gallium nitride layer 12 is grown on the base substrate 10, which may be formed of sapphire, or silicon carbide, or silicon, and will be referred to as a heterogeneous substrate. However, since gallium nitride and sapphire have differences in the lattice constant and the coefficient of thermal expansion, bending may occur as illustrated in FIG. 1. Moreover, when the gallium nitride layer is separated from the heterogeneous substrate, a phenomenon, such as crystallographic tilting, occurs in the environment in that the gallium nitride layer 12 is grown as illustrated in FIG. 2. Threading dislocation occurs in high-density in the gallium nitride film, thereby affecting planarization of the surface and causing the bending.
In conventional methods to minimize the aforementioned bending, the size of a particle of an abrasive, which is used for processing the surface, is reduced by steps, so that a damaged layer of the surface is removed. Polishing equipment may be used for flattening one side or both sides of the gallium nitride substrate which is bent due to the use of the heterogeneous substrate. However, even though the surface of the gallium nitride substrate is polished by the aforementioned methods, since both sides thereof are different from each other in the extent of damage, a strain difference occurs on both sides. As a result, the substrate is bent to be concave toward the side on which the extent of damage is less. The bending of the gallium nitride layer and cracking of the gallium nitride layer due to the bending are disadvantageous to mass production yields and cost, thereby decreasing the efficiency of production.